1. Field of the Invention
This invention relates to a focus detecting device using one or more arrays of photo-sensors and more particularly to an improvement in a focus detecting device of a type wherein focus detection is performed by receiving one or more images of an object formed through image forming optical means with one or more arrays of many electric charge storing type photo-sensors, or one or more photo-sensor arrays, which operate in an electric charge storing mode; then, by extracting, the output of each of the photo-sensors in a time serial manner, to electrically scan the image or images received by them; and then by processing the output or outputs of such scanning in a suitable manner.
2. Description of the Prior Art
In a focus detecting device of a type well known in the art, an image forming lens forms an image of an object on a photo-electric transducing element which changes its output (an internal resistance value) according as the sharpness of the image changes and, in a process of moving the lens along its optical axis, a just focus is attained when the output (the internal resistance value) of the photo-electric transducing element has reached its extreme value.
In another type of focus detecting device which is also well known in the art, two images of an object are formed respectively on different photo-electric transducing elements through a range finding optical system, with apparent parallax corresponding to the distance to the object, and, in a process of shifting one image toward the other in response to a focus adjusting operation of an objective lens, a just focus is attained when the outputs (internal resistance values) of the two transducing elements have come to coincide with each other or, in other words, when the positions of the two images have come to coincide with each other.
In the meantime, rapid advancement of semiconductor technology has made commerically available, at relatively low prices, photo-sensor arrays, such as a photo-diode array (MOS-image sensor), CCD (a charge coupled device), BBD (a bucket brigade device), etc. The photo-sensor array, such as a CCD or a BBD is composed of many photo-sensors of an electric charge storing type. The output of each of these charge storing photo-sensors can be taken out one after another in a time serial manner by applying a start pulse and scanning pulses through a driver circuit. Accordingly, with such a photo-sensor array employed, a video signal accurately representing an image of an object formed by an image forming lens is obtainable by scanning, in a purely electrical manner, the image of the object, so that focus detection can be accomplished with higher accuracy thereby.
Therefore, in view of such an advantageous function of a photo-sensor array, there have been made many attempts to utilize such a photo-sensor array for focus detection.
As for an example of an improvement over the former type of the two conventionally known types of focus detecting devices mentioned in the foregoing, a device has been proposed by the same assignee of the present invention under U.S. patent application Ser. No. 563,462, filed on Mar. 31, 1976, entitled "A SYSTEM FOR EXPOSURE MEASUREMENT AND/OR FOCUS DETECTION BY MEANS OF IMAGE SENSOR". In this device, an image of an object formed by an image forming lens on an area type photo-sensor array (a matrix sensor array) and, under such a condition, the outputs of the photo-sensors are taken out in time series and then are converted into digital values one after another by an analog-to-digital convertor. Then, the difference between the digital converted output signals of a pair of sensors which are adjoining each other in the array are obtained in absolute value by means of a subtracter. The difference signals thus obtained from all of the sensors in the sensor array are accumulated by an accumulator in one scanning operation and the results of the accumulation is used as information on image sharpness.
And as for an example of improvement over the latter of the two types of conventional focus detecting devices mentioned in the foregoing, a device has been proposed by the same assignee of the present invention for U.S. Pat. No. 3,898,676, entitled "DISTANCE DETECTING DEVICE". In this device, two images of an object are formed through a range finding optical system with apparent parallax corresponding to the distance to the object and are respectively received by different photo-sensor arrays. Under such a condition, the output of each sensor is simultaneously taken out by time seriating means in a time serial manner. After these outputs are sampled and held by means of a sample-and-hold circuit, they are converted by a low pass filter into wave form signals. Then, the phase difference between the outputs of these photo-sensor array is detected by means of a phase discriminator. A motor is actuated by the output of the phase discriminator to shift objective lens means along its optical axis. Then, in response thereto, one of the two images is shifted toward the other and thus a just focus is attained when the phase difference between the wave form signals from the two sensor arrays becomes zero or, in other words, when the positions of the images relatively come to coincide with each other.
On the other hand, when irradiated with light, the electric charge storing type photo-sensors that constitute a photo-sensor array, such as a CCD or a BBD are capable of storing electric charge proportional to the integrated amount of the incident light (intensity of light.times.time) at a speed corresponding to the intensity of the incident light. Generally, such a photo-sensor is applied in such a manner that, after electric charge is discharged, electric charge corresponding to the integrated amount of incident light received during a period of time before next discharge is stored therein. Accordingly, the above stated period of time is regarded as electric charge storing time of a photo-sensor. However, a storable quantity of electric charge is a photo-sensor, i.e. an electric charge saturation level, is predetermined and, as generally known, after such a saturation level is exceeded, an excess of electric charge flows out to mix in other photo-sensors. This is known as a "blooming" effect, which makes it hardly possible to obtain correct video signals.
In addition to such a problem, when the quantity of the electric charge stored in a photo-sensor is extremely small, the S/N ratio (signal to noise ratio) against a dark current and the like becomes too low for obtaining a correct picture element signal.
Where a photo-sensor array comprising such electric charge storing type photo-sensors is used for a focus detecting device, therefore, the electric charge stored in each photo-sensor must be controlled so as not to exceed its saturation level irrespective of variation in the brightness of an object and, also for making the S/N ratio sufficiently high for unfailingly obtaining a correct video signal, the integrated amount of light to be received by the photo-sensor must be adjusted to a suitable degree. Without such arrangement, it is hardly possible to make focus detection with high accuracy using such a photo-sensor array.